Onion-derived activated carbons with enhanced surface area for improved hydrogen storage and electrochemical energy...

High surface area activated carbons (ACs) were prepared from a hydrochar derived from waste onion peels. The resulting ACs had a unique graphene-like nanosheet morphology. The presence of N (0.7%) and O content (8.1%) in the OPAC-800 °C was indicative of in situ incorporation of nitrogen groups from the onion peels. The specific surface area and pore volume of the best OPAC sample was found to be 3150 m² g^-1 and 1.64 cm³ g^-1, respectively. The hydrogen uptake of all the OPAC samples was established to be above 3 wt% (at 77 K and 1 bar) with the highest being 3.67 wt% (800 °C). Additionally, the OPAC-800 °C achieved a specific capacitance of 169 F g^-1 at a specific current of 0.5 A g^-1 and retained a capacitance of 149 F g^-1 at 5 A g^-1 in a three electrode system using 3 M KNO₃. A symmetric supercapacitor based on the OPAC-800 °C//OPAC-800 °C electrode provided a capacitance of 158 F g^-1 at 0.5 A g^-1. The maximum specific energy and power was found to be 14 W h kg^-1 and 400 W kg^-1, respectively. Moreover, the device exhibited a high coulombic efficiency of 99.85% at 5 A g^-1 after 10 000 cycles. The results suggested that the high surface area graphene-like carbon nanostructures are excellent materials for enhanced hydrogen storage and supercapacitor applications.